In the construction of a thick film hybrid circuit, it is generally desirable to position the circuit components of the circuit as close to each other as possible, resulting in a higher component density, so as to minimize the size of the hybrid circuit. As shown in FIG. 1, one known method for achieving a high component density for a thick film hybrid circuit 110 is to place conductors, which make electrical interconnects between circuit devices 22a-c and 24, within a multilayer structure composed of layers of metal runners 14 interlaid with layers 16 of an electrically insulating, or dielectric, material. Successive layers of metal runners 14 are electrically insulated from each other with an intermediate layer 16 of the dielectric material, with metallized holes, or vias 18, being provided to electrically interconnect appropriate metal runners 14 with the circuit devices' corresponding bond pads 20 on the surface of the multilayer structure. Metallized vias 18 are also provided to electrically interconnect successive layers of metal runners 14 where necessary. The multilayer structure is then adhered to the surface of a suitable substrate 12 which provides the structural support for the hybrid circuit 110. A heat sink 26 is typically adhered to the lower surface of the substrate 12 by which heat generated by the circuit devices 22a-c and 24 is conducted away from the hybrid circuit 110.
While the above approach is highly desirable from the standpoint of maximizing component density, significant disadvantages exist. A primary disadvantage is that heat generated by the circuit devices 22a-c and 24 must be conducted through the multilayer structure, which typically will have a higher thermal resistance than the substrate 12 and heat sink 26. Consequently, heat is not as readily conducted away from the circuit devices 22a-c and 24, resulting in higher operating temperatures. Furthermore, when using certain bonding technologies, such as soldering, adhesion of the circuit devices 22a-c and 24 to the exposed dielectric layer 16 of the multilayer structure is not as reliable as directly attaching the circuit devices 22a-c and 24 to the substrate 12.
Yet another shortcoming with the hybrid circuit 110 of the prior art is the inherent structural weakness of the dielectric layers 16, particularly when subjected to stresses induced during thermal cycling of the hybrid circuit 110. Thermal gradients resulting from heat generated by large surface mounted devices, such as flip chip integrated circuits 22a, chip-and-wire integrated circuits 22b, and capacitors, resistors and inductors 22c, have a tendency to create cracks 28 through the dielectric layers 16, which has the potential for creating an open circuit within the hybrid circuit 110.
Accordingly, what is needed is a thick film hybrid circuit which utilizes a multilayer conductor-dielectric structure in order to enhance the component density of the hybrid circuit, while also overcoming the significant disadvantages and shortcomings of the prior art. For example, such a hybrid circuit would promote the conduction and diffusion of heat generated by the circuit devices, enable a more reliable method for attaching the circuit devices, and reduce the thermal gradients which promote the formation of cracks within the dielectric material used in the multilayer structure by thermally insulating the multilayer structure from large circuit devices.